It is oftentimes desirable to fracture boreholes in order to increase or restore the permeability of fluids such as oil, gas or water into the borehole thereby increasing the production of oil, gas, and/or water from the borehole. Hydraulic fracturing is a technique commonly used in the oil industry to create fractures that extend from an oil borehole into rock. Such fracturing is accomplished by injecting a suitable fracturing fluid within the borehole. Thereafter, sufficient pressure is applied to the fracturing fluid in order to cause the formation to break down with the attendant formation of one or more fractures therein. Simultaneously with or subsequent to the formation of the fracture a suitable carrier fluid having suspended therein a propping agent or proppant such as sand or other particulate material is introduced into the fracture to hold the fracture open after the fluid pressure is released. Typically, the fluid containing the proppant is of a relatively high viscosity in order to reduce the tendency of the propping agent to settle out of the fluid as it is injected down the well and into the fracture.
Hydraulic fracturing methods are disclosed in U.S. Pat. Nos. 3,965,982; 4,067,389; 4,378,845; 4,515,214; 4,549,608 and 4,685,519, for example. Hydraulic fracturing is sometimes performed on very thick pays. As a result, fractures are induced in stages along the length of a borehole, creating multiple reservoir zones along the borehole.
The extent of hydraulic fracturing and the location of proppant materials is currently diagnosed by the use of radioactive tracers as described in U.S. Pat. No. 3,987,850. Typically, radioactive tracers with discriminating gamma energy signatures are displaced into the various stages of a fracturing operation at predetermined activities that can be measured using multi-spectral gamma ray tools used in wireline logging operations. The conventional method of introducing radioactive tracers into the fracturing fluids is by surface injection. This allows for the determination of various subsurface zones in affected intervals that have been tagged.
The use of radioactive tracer materials for tracing subsurface zone location from hydraulic fracturing operations poses a high risk from a health safety and environment (“HSE”) prospective. The risk of dispersing radioactive material is high with respect to uncontrolled variables such as equipment failure leading to the release of a radioactive tracer material, or the retention of radioactive tagged fracturing fluids in piping and blending or well head equipment, either by mechanical deposition or chemical reaction leading to fixed or loose radioactive contamination of the exposed items. The presence of radioactive materials and contamination in the environment leads to pollution and burdens from exposure of gamma/beta emitting radioisotopes to people and anything in close proximity to them.
What is needed is a new method and compositions that allow fracture and other borehole operation diagnostics to be performed with small or no risk from an HSE perspective from both initial surface injection operations, exposure to equipment and from the recovery of tagged effluents when the well is flowed back.